I drove to a neighbouring track.
I'm glad I did as I last did track speed sessions in 2003.
Real eye opener and back to school.
10 x 200 m @ 47,6 sec average and 200 metre jog.
In my comfort zone but will pace much better next track session which is 10 x 400 m coming up on Tuesday.

you should go through your previous training cycles and pulse ranges to see how and when the 'blip' occurred, so that if you have some early warning signs, you can do things to avoid it happening again in the future.

running is every much about confidence, and blips can often cause havoc

In all 3 cycles on the pace runs and park run's my speed endurance goes haywire at the 3km mark. I'm on pace and then slow down perceptibly for the remainder of the 5k distance.

I went sub 20 on numerous times in 2017 with my own schedule and the weekly hilly long run was I think a key workout with a weekly tempo session and a 5 x 5 minute fartlek type of session.

The difference is that I dont feel as holistically revitalized as with your program structure in comparison to my ówn previous run as you feel, weekly racing / gung ho method.

However, with my previous schedule in 2017 my wheels never came off either in the last 2k's thereforesome tweaking is currently needed.

For week 1 I have only been doing 3 x 1 mile - maybe I should increase this?
For week 2 I have only been doing 4 x 1km - maybe I should increase this?
As for introducing a weekly long run this wont seem to fit into your structure without disturbing the quality days and subsequent rest.

I have also focused on dropping my weight down from 87.5 kg to 85kg.
Height 178cm - age 52.

By week 3 I am raring to go, a bundle of energy but the wheels always seem to come off in the last 2 km even though I am pacing myself correctly from the word GO.